Semiconductor device and method for fabricating same

ABSTRACT

A semiconductor device includes a semiconductor substrate having a trench in its surface, an insulating film in the trench, a doped conductive layer on the insulating film, a gate insulation film and a gate electrode on the doped conductive layer over the trench, and source and drain impurity regions in the surface of the semiconductor substrate at sides of the gate electrode.

[0001] This application claims the benefit of Korean patent application No. 9235/1999, filed Mar. 18, 1999, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a semiconductor device, and more particularly, to a semiconductor device and a method for fabricating the same with an improved short channel effect and reduced punch-through caused during channel formation.

[0004] 2. Discussion of the Related Art

[0005] A related art semiconductor device and method for fabricating the same will be explained with reference to the attached drawings.

[0006] Referring to FIG. 1, a related art semiconductor device includes a semiconductor substrate 11, a gate insulating film 12 on the semiconductor substrate 11, and a gate electrode 13 a on the gate insulating film 12. Insulating sidewalls 16 are formed at both sides of the gate electrode 13 a, and source and drain impurity regions 17 of an LDD (Lightly Doped Drain) structure are formed in a surface of the semiconductor substrate 11 on both sides of the gate electrode 13 a.

[0007] FIGS. 2A-2D illustrate cross-sections showing a related art method for fabricating the semiconductor device of FIG. 1.

[0008] Referring to FIG. 2A, the related art method for fabricating a semiconductor device first forms the gate insulating film 12 and a polysilicon film 13 on the semiconductor substrate 11. A photoresist 14 is coated on the polysilicon film 13, and subjected to patterning after exposure and development, to define a gate region.

[0009] As shown in FIG. 2B, the patterned photoresist 14 is used as a mask to selectively remove the polysilicon film 13 and the gate insulating film 12, forming the gate electrode 13 a.

[0010] As shown in FIG. 2C, the patterned photoresist 14 is removed, and the gate electrode 13 a is used as a mask when n-type impurity ions are doped into an entire surface of the semiconductor substrate 11, forming LDD regions 15 in the surface of the semiconductor substrate 11 at both sides of the gate electrode 13 a.

[0011] As shown in FIG. 2D, an insulating film is formed on an entire surface of the semiconductor substrate 11, including the gate electrode 13 a, and etched back, forming the insulating sidewalls 16 at both sides of the gate electrode 13 a. Then, the gate electrode 13 a and the insulating sidewalls 16 are used as a mask to heavily dope n-type impurity ions into the entire surface of the semiconductor substrate 11, forming the source and drain impurity regions 17 in the surface of the semiconductor substrate 11 on both sides of the gate electrode 13 a.

[0012] However, the related art semiconductor device and the method for fabricating the same have the following problem. Due to higher density of devices, a channel becomes shorter following a scaling-down of the gate electrode. DIBL (Drain Induced Barrier Lowering) characteristic of the transistor is significantly lowered, which likely generates an electron punch-through.

SUMMARY OF THE INVENTION

[0013] Accordingly, the present invention is directed to a semiconductor device and a method for fabricating the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

[0014] An object of the present invention is to provide a semiconductor device and a method for fabricating the same that improve the short channel effect, as well as to reduce punch-through caused during channel formation.

[0015] Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0016] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect of the present invention there is provided a semiconductor device including a semiconductor substrate having a trench in its surface, an insulating film in the trench, a doped conductive layer on the insulating film, a gate insulation film and a gate electrode on the doped conductive layer over the trench, and source and drain impurity regions in the surface of the semiconductor substrate at sides of the gate electrode.

[0017] In another aspect of the present invention, there is provided a method for fabricating a semiconductor device, the method including the steps of forming a first trench in a surface of a semiconductor substrate, burying an insulating film in the first trench, selectively removing the insulating film to form a second trench in the surface of the semiconductor substrate, burying a doped conductive layer in the second trench, forming a gate insulation film and a gate electrode on the doped conductive layer, and forming source and drain impurity regions in the surface of the semiconductor substrate at sides of the gate electrode.

[0018] In another aspect of the present invention, there is provided a semiconductor device including a semiconductor substrate, a trench in a surface of the semiconductor substrate, an insulating layer over the trench, a doped polysilicon layer on the insulating layer, a gate insulation layer on the doped polysilicon layer over the trench, a gate electrode on the doped polysilicon layer, and source and drain impurity regions in the surface of the semiconductor substrate at sides of the gate electrode.

[0019] In another aspect of the present invention, there is provided a method for fabricating a semiconductor device, the method including the steps of forming a first trench in a surface of a semiconductor substrate, forming an insulating layer in the first trench, selectively removing the insulating layer to form a second trench in the surface of the semiconductor substrate, forming a doped polysilicon layer in the second trench, forming a gate insulation layer and a gate electrode on the doped polysilicon layer, and forming source and drain impurity regions in the surface of the semiconductor substrate at sides of the gate electrode.

[0020] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

[0021] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

[0022] In the drawings:

[0023]FIG. 1 illustrates a cross-section of a related art semiconductor device;

[0024] FIGS. 2A-2D illustrate cross-sections showing the steps of a related art method for fabricating a related out semiconductor device;

[0025]FIG. 3 illustrates a cross-section of a semiconductor device in accordance with a preferred embodiment of the present invention; and

[0026] FIGS. 4A-4H illustrate cross-sections of the steps of a method for fabricating a semiconductor device in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0027] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[0028] Referring to FIG. 3, a semiconductor device in accordance with a preferred embodiment of the present invention includes a gate electrode 29 a formed over a gate insulating film 28 and over a region of a semiconductor substrate 21. A doped polysilicon layer 27 is formed under the gate electrode 29 a up to a surface of the semiconductor substrate 21. A second insulating film 25 is beneath the doped polysilicon layer 27. Insulating sidewalls 32 are at both sides of the gate electrode 29 a. Source and drain impurity regions 33 of an LDD structure are formed in a surface of the semiconductor substrate 21 at both sides of the gate electrode 29 a. The doped polysilicon layer 27 is a channel region of a transistor, with channel depth based on a thickness of the doped polysilicon layer 27.

[0029] FIGS. 4A-4H illustrate cross-sections of a method of fabricating a semiconductor device in accordance with the preferred embodiment of the present invention.

[0030] Referring to FIG. 4A, a first insulating film 22 is formed on the semiconductor substrate 21, and selectively etched to expose a portion of the semiconductor substrate 21. The first insulating film 22 is formed of an oxide, a nitride, or a stack of an oxide and a nitride. Then, the first insulating film 22 is used as a mask to selectively remove a portion of the exposed semiconductor substrate 21, forming a first trench 23.

[0031] As shown in FIG. 4B, the first insulating film 22 is removed, and the second insulating film 24 is formed on an entire surface of the semiconductor substrate 21, including the first trench 23, and planarized by CMP (Chemical Mechanical Polishing) or etched back, leaving the second insulating film 24 only in the first trench 23. The second insulating film 24 may be formed of an oxide or a nitride.

[0032] As shown in FIG. 4C, a third insulating film 25 is formed on an entire surface of the semiconductor substrate 21 and subjected to photolithography and etching, selectively removing the third insulating film 25 to expose a surface of the second insulating film 24. The third insulating film 25 may be formed of an oxide, a nitride, or a stack of an oxide and a nitride. Then, the third insulating film 25 is used as a mask to selectively remove the second insulating film 24, forming a second trench 26. A channel region will be formed in the portion selectively removed from the second insulating film 24.

[0033] As shown in FIG. 4D, the third insulating film 25 is removed, and the doped polysilicon layer 27 is formed on an entire surface of the semiconductor substrate 21 including the second trench 26. P-type impurity doped polysilicon is used as the doped polysilicon layer 27 to fabricate an NMOS transistor, and N-type impurity doped polysilicon is used to fabricate a PMOS transistor as the doped polysilicon layer 27. Alternatively, undoped polysilicon may be used, and N-type or P-type impurity ions may be doped thereafter. A planarization is conducted by CMP or etching back, to leave the doped polysilicon layer 27 only in the second trench 26. The doped polysilicon layer 27 remaining in the second trench 26 is used as the channel region. Therefore, a depth of the channel region can be adjusted freely according to a depth of the second trench 26.

[0034] As shown in FIG. 4E, the gate insulating film 28 is formed on the semiconductor substrate 21, and the second polysilicon layer 29 (for a gate electrode) is formed on the gate insulating film 28. Then, a photoresist 30 is coated on the second polysilicon layer 29, and subjected to patterning after exposure and development, leaving the photoresist 30 only over the first and second trenches 23 and 26, thus defining a gate region.

[0035] As shown in FIG. 4F, the patterned photoresist 30 is used as a mask to selectively remove the second polysilicon layer 29 and the gate insulating film 28, forming a gate electrode 29 a.

[0036] As shown in FIG. 4G, the photoresist 30 is removed, and the gate electrode 29 a is used as a mask in lightly doping n-type impurity ions into an entire surface of the semiconductor substrate 21, forming LDD regions 31 in the surface of the semiconductor substrate 21 on both sides of the gate electrode 29 a.

[0037] As shown in FIG. 4H, an insulating film is formed on the entire surface of the semiconductor substrate 21, including the gate electrode 29 a, and etched back to form insulating sidewalls 32 at both sides of the gate electrode 29 a. Then, the gate electrode 29 a and the insulating sidewalls 32 are used as a mask in heavily doping n-type impurity ions into the entire surface of the semiconductor substrate 21, forming source and drain impurity regions 33 of an LDD structure in the surface of the semiconductor substrate 21 on both sides of the gate electrode 29 a.

[0038] The semiconductor device and the method for fabricating the same of the present invention have the following advantages.

[0039] First, the provision of an insulating film between a source and a drain in a substrate improves the short channel effect.

[0040] Second, a depth of a channel region can be freely adjusted.

[0041] Third, the occurrence of punch-through caused by a depletion layer between a source and a drain during channel formation can be prevented.

[0042] It will be apparent to those skilled in the art that various modifications and variations can be made in the semiconductor device and the method for fabricating the same of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A semiconductor device comprising: a semiconductor substrate having a trench in its surface; an insulating film in the trench; a doped conductive layer on the insulating film; a gate insulation film and a gate electrode on the doped conductive layer over the trench; and source and drain impurity regions in the surface of the semiconductor substrate at sides of the gate electrode.
 2. The semiconductor device of claim 1, wherein the source and drain impurity regions form a lightly doped drain (LDD) structure.
 3. The semiconductor device of claim 1, wherein the insulating film is below a surface of the semiconductor substrate.
 4. The semiconductor device of claim 1, wherein the doped conductive layer is level with a surface of the semiconductor substrate.
 5. The semiconductor device of claim 1, further including insulating sidewalls formed at the sides of the gate electrode.
 6. The semiconductor device of claim 1, wherein the doped conductive layer is formed of polysilicon.
 7. The semiconductor device of claim 1, wherein the doped conductive layer comprises a channel region.
 8. The semiconductor device of claim 1, wherein the insulating film is formed of one of an oxide and a nitride.
 9. A method for fabricating a semiconductor device, the method comprising the steps of: forming a first trench in a surface of a semiconductor substrate; burying an insulating film in the first trench, selectively removing the insulating film to form a second trench in the surface of the semiconductor substrate; burying a doped conductive layer in the second trench; forming a gate insulation film and a gate electrode on the doped conductive layer; and forming source and drain impurity regions in the surface of the semiconductor substrate at sides of the gate electrode.
 10. The method of claim 9, wherein a depth of the second trench is adjusted to control a depth of a channel region.
 11. The method of claim 9, wherein the step of burying a doped conductive layer further includes the steps of: forming the doped conductive layer on the surface of the semiconductor, including the second trench; and planarizing the surface of the semiconductor substrate and the doped conductive layer.
 12. The method of claim 11, wherein the planarizing step includes one of Chemical Mechanical Polishing (CMP) and etching back.
 13. The method of claim 9, wherein the doped conductive layer is formed of polysilicon
 14. The method of claim 9, further including the step of forming insulating sidewalls at the sides of gate electrode.
 15. The method of claim 9, wherein the step of burying the doped conductive layer comprises the steps of forming an undoped conductive layer and selectively doping impurity ions thereto.
 16. The method of claim 9, wherein the insulating film is formed of one of an oxide and a nitride.
 17. The method of claim 9, wherein the gate electrode is formed of polysilicon.
 18. A semiconductor device comprising: a semiconductor substrate; a trench in a surface of the semiconductor substrate; an insulating layer over the trench; a doped polysilicon layer on the insulating layer; a gate insulation layer on the doped polysilicon layer over the trench; a gate electrode on the doped polysilicon layer; and source and drain impurity regions in the surface of the semiconductor substrate at sides of the gate electrode.
 19. The semiconductor device of claim 18, wherein the source and drain impurity regions form a lightly doped drain (LDD) structure.
 20. The semiconductor device of claim 18, wherein the insulating layer is entirely below the surface of the semiconductor substrate.
 21. The semiconductor device of claim 18, wherein the doped polysilicon layer is level with the surface of the semiconductor substrate.
 22. The semiconductor device of claim 18, further including insulating sidewalls formed at the sides of the gate electrode.
 23. The semiconductor device of claim 18, wherein the insulating layer is formed of one of an oxide and a nitride.
 24. A method for fabricating a semiconductor device, the method comprising the steps of: forming a first trench in a surface of a semiconductor substrate; forming an insulating layer in the first trench; selectively removing the insulating layer to form a second trench in the surface of the semiconductor substrate; forming a doped polysilicon layer in the second trench; forming a gate insulation layer and a gate electrode on the doped polysilicon layer; and forming source and drain impurity regions in the surface of the semiconductor substrate at sides of the gate electrode.
 25. The method of claim 24, wherein a depth of the second trench is adjusted to control a depth of a channel region.
 26. The method of claim 24, wherein the step of forming a doped polysilicon layer includes the steps of: forming the doped polysilicon layer on the surface of the semiconductor, including the second trench; and planarizing the surface of the semiconductor substrate and the doped polysilicon layer.
 27. The method of claim 26, wherein the planarizing step includes one of Chemical Mechanical Polishing (CMP) and etching back.
 28. The method of claim 24, further including the step of forming insulating sidewalls at the sides of gate electrode.
 29. The method of claim 24, wherein the step of forming a doped polysilicon layer comprises the steps of forming an undoped polysilicon layer and selectively doping impurity ions thereto.
 30. The method of claim 24, wherein the insulating layer is formed of one of an oxide and a nitride.
 31. The method of claim 24, wherein the gate electrode is formed of polysilicon. 